In laying line pipes (straight pipes, bent pipes and the like for use in a pipeline is collectively referred to as “line pipe(s)”) for transporting fluids such as petroleum or natural gas, not only straight pipes but also bent pipes are used. Generally, the bent pipes are joined to the straight pipes by circumferential welding at exact sites of laying line pipes.
It is preferable that the straight pipe and the bent pipe to be joined together are identical in inside diameter and the inside surface around the circumferentially welded area between the straight pipe and the bent pipe is flush and has no step (difference in height level) so that fluids such as petroleum or natural gas may flow smoothly through the inside of the pipelines. Therefore, it is preferable that the straight pipe and the bent pipe are identical in inside diameter as much as possible and, for that purpose, it is necessary to produce both straight pipes and bent pipes with high dimensional accuracy.
Bent pipes are generally manufactured by hot or cold bending of straight pipes. However, on the occasion of bending, the inner-radius side of a bent section becomes thicker in wall thickness and an outer-radius side of the bent section becomes thinner in wall thickness and, therefore, it is not easy to make a pipe end inside diameter of the bent pipe identical to a pipe end inside diameter of the straight pipe.
Furthermore, seamless pipes for line pipes are produced while primarily controlling an outside diameter and a wall thickness thereof, so that it is difficult to produce pipes, even when they are straight, that have no variations in inside diameter thereof. Therefore, it is much more difficult to produce a bent pipe with high dimensional accuracy by bending such a seamless pipe which is straight (hereinafter referred to as “seamless straight pipe”) (such a bent pipe is hereinafter referred to as “seamless bent pipe”).
Japanese Patent Application Publication No. 2004-223530 proposes a method for inhibiting bellows defective or flattening defective from appearing on the occasion of hot or cold bending of a metallic straight pipe. However, this method is insufficient for controlling inside and outside diameters of a bent pipe to attain the targeted dimensions.
FIG. 1 shows a configuration of a hot bending apparatus referred to as the prior art in Japanese Patent Application Publication No. 2004-223530 and an example of a bent pipe manufactured by bending. FIG. 1A shows a transitional state of bending by the bending apparatus, FIG. 1B shows a plan view of the bent pipe manufactured, and FIG. 1C shows a sectional view of the bent pipe as seen in the direction of arrows II-II in FIG. 1B.
As shown in FIG. 1A, a straight pipe 1 to be bent is passed through an induction heating coil 2, one end of the straight pipe 1 is securely held by a front clamp 4 mounted on a bending arm 3 that swings around a fulcrum O as the center of swing and the other end thereof is tightly held by a rear clamp 5, and an intermediate portion is pinched by guide rollers 6. A short pipe segment, in a pipe axis direction, of the straight pipe 1 is induction-heated to a temperature allowing ready plastic deformation by means of the induction heating coil 2 and, at the same time, the rear clamp 5 is pushed forward in a pipe axis direction by means of a driving system (not shown) for the straight pipe 1 to continuously advance in a pipe axis direction.
Then, while the short pipe segment, i.e. a heated zone 7 heated by the induction heating coil 2 is continuously forwardly moved along a pipe axis direction, the heated zone 7 is given a bending moment resulting from the swing of the bending arm 3 and is thereby subjected to bending deformation and, immediately thereafter, the bent portion is cooled by spraying with a cooling medium 8, such as cooling water, from the induction heating coil 2, whereby the affected portion after bending is hardened.
In this manner, the bent pipe comprising a bent section 1a and straight pipe sections 1b, 1c on both sides thereof, as shown in FIG. 1B, is manufactured. The thus-manufactured bent pipe shows a thicker wall thickness on an inner-radius side of the bent section 1a and a thinner wall thickness on an outer-radius side of the bent section 1a, as shown in FIG. 1C.